Vancouver Avian Research Centre

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Species: Brown Headed Cowbird Molothrus ater

Description:

Brown-headed Cowbirds are easily recognized as members of the blackbird family (Icterids). They are identified by a dark brown head on a black body that has a green iridescent sheen. The bill is straight, pointed and looks relatively short and thick.
Gregarious birds, they are often found in mixed flocks with other blackbirds and starlings. These flocks can number in the hundreds of thousands in the fall and winter after the breeding season.
During breeding season their range is widespread across North America, spending winters in the Southeastern US and Mexico. They have a somewhat dubious claim to fame as the most well-known North American practitioners of brood parasitism, where the young are foisted off into another species’ nest to be hatched and raised.

General: Adults are about 19 cm long, and weigh about 44 grams, with short tails and pointed wings. Straight, stout bill, some say it is almost finch-like.

Adult Male: Black body with a glossy green sheen, brown head.

Adult Female: Gray-brown overall, with a paler head and underparts. The breast has fine, faint streaks and the female has a dark eye.

Juvenile: Gray-brown, back feathers with a pale fringe giving the back a scaly appearance. Underparts paler, with distinct fine streaks on the breast.

Similar Species: Bronzed Cowbirds are shorter-tailed and stockier with a thick ruff of feathers around the neck; Shiny Cowbirds have slenderer bills and are a uniform glossy black. (Both are rare in the Pacific Northwest.) Brewer’s Blackbirds can be distinguished by a glossy blue-black head and a yellow eye.

Behaviour: Roost colonially, tend to congregate in large mixed flocks in open fields. They eat grain and insects, foraging on the ground. Flight is direct with constant wingbeats.

Habitat: Tend to congregate in open field, often near livestock, but will hang around forest edges during breeding season to utilize host nests. Habitat can vary with time of day, with breeding behavior taking place in the morning (near woodland nests) and foraging in open fields happening later in the day.

Unable to build their own nests, Brown-headed Cowbirds rely on other birds to hatch and rear their young by laying eggs in other species’ nests. When placing her egg in a host nest, a female cowbird will often peck or dislodge a host egg. A female can lay as many as 40 eggs in a season. While the young cowbirds may not oust host eggs and young from the nest per se, cowbird eggs do hatch sooner, grow faster and put a lot of pressure on the host young and parent by competing for food. If the host species is larger them the cowbirds, the host species is usually able to bear the burden, but in smaller species the young are usually out-competed and starve. For this reason cowbirds are considered to be a contributing factor to the decline of some songbird species. They lay their eggs in the nests of over 220 species, with the most common being the Yellow Warbler, Song and Chipping sparrows, Red-eyed Vireo, Eastern and Spotted towhees, and Red-winged Blackbird.

Conservation status:

Originally associated with American Bison, the Brown-headed Cowbird’s range has increased with the clearing of fields and the introduction of livestock. It is a species of lowest concern, having thrived with human influence beginning in the 1800s as forests were cleared and the spread of agricultural lands increased its feeding habitat.

Sources:
The Sibley Guide to Bird Life and Bird Behavior– David Sibley
The Sibley Guide to Birds – David Sibley
The Birder’s Handbook – Paul Erlich
All About Birds, Cornell Lab of Ornithology: www.allaboutbirds.org
 
Capture Rates


As a brood parasite, Brown-headed Cowbird capture rate (2010-2012; standardized as birds captured per 100 net hours) peaks in mid-summer during the breeding season. Cowbirds are short distance migrants, moving south for the winter as seen by our zero capture rate between September - March.

 

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